On top of your primary firearm, every survivor should have a side arm, and a melee weapon for when things get really bad. Side arms should probably be a good semi-auto handgun, preferably of the 9mm or .45 variety. Because use of this gun should be of last resort, you want to make sure that one well placed center mass shot will be enough to put the bodies to the floor in the event of catastrophe. You would want to make sure you have plenty of additional mag cartridges that you can keep loaded and on your person at all times, because with lots of ammo, and quick reload speeds, handguns are an invaluable tool in close quarters environments. They are light, compact, and maneuverable, and as such, they might be one of the best weapons to have at your disposal. I would have to do more research on varieties of modern handguns before i could make a good judgment on which ones are, and aren't good firearms, but so far as I'm concerned, most of the choice comes down to personal preference and which brand you support the most. Personally, I believe that most any Glock or Barretta would be a sound choice, as they are the primary side arms of law enforcement and military organizations, and have been for a good while.
If anybody in my party during the apocalypse shows up with anything like a .44 Magnum, I'm going to slap them... Guns like this are completely unnecessary, and are totally impractical. They are difficult to use, due to massive recoil, and only hold 6 rounds at a time. However, because of the simple mechanics of revolvers, they are probably more reliable, and you can keep your ammo topped off all the time. I myself wouldn't carry a revolver by itself, but it might be a good addition to a more modern handgun, adding to the versatility of your loadout.